1. Technical Field of the Invention
The present invention relates to a motorcycle frame structure. More particularly, the present invention relates to a body frame of a motorcycle in which a pivot plate, vertically and swingably supporting a front portion of a swing arm which is rotatably supporting a rear wheel at a rear portion, is provided at a rear portion of each of a plurality of main frames extending rearward and downward from a head pipe which is steerably supporting a front fork and a steering handlebar, and a front end of a seat rail is fastened to an upper portion of the pivot plate at two positions of a front end vertically spaced apart from each other.
2. Background Art
A conventional motorcycle body frame is known in which two upper and lower front ends of the seat rail are laterally put on the rear portion of a pivot plate and are fastened with a bolt to the pivot plate from the left-right direction outside of the body frame. A frame of this known design is described in Japanese Laid-open Patent No. 2003-81166.
Incidentally, the fastened portion of the seat rail to the pivot plate needs strength to withstand a load applied to the seat rail from above. As disclosed in Japanese Laid-open Patent No. 2003-81166 mentioned above, the configuration in which the two upper and lower front ends of the seat rail are fastened to the pivot plate from the left-right direction outside of the body frame needs to increase the rigidity of a boss attached to the pivot plate in order to ensure the strength of such a fastened portion. This increases the thickness of the attachment boss, which leads to an increase in the weight of the pivot plate.
Although the known motorcycle frame has some utility for its intended purposes, a need still exists in the art for an improved motorcycle frame structure. In particular, there is a need for an improved motorcycle frame structure in which a front end of a seat rail can firmly be fastened to a pivot plate while preventing an increase in the weight of the pivot plate.